Signals transduced by the precursor B cell receptor (preBCR) are necessary for B cell development. Humans and mice genetically deficient in preBCR components suffer from an absence or paucity of pre-B and more mature B cells that is responsible for a state of agammaglobulinemia and resultant severe immune deficiency. Deregulating or loss-of-function mutations of downstream preBCR signal transduction molecules are associated with B cell leukemia and lymphoma. An enigmatic aspect of preBCR signaling is the physiological means by which receptor signaling is activated. Evidence for both ligand-dependent and ligand-independent mechanisms has been demonstrated in cell culture systems and suggested to be physiological. Common to both models has been the idea that receptor localization to the cell surface is critical, either to engage ligands or key signal transduction molecules that reside there, because surface expression has directly correlated with receptor activity. Contrary to this model, we have recently demonstrated that preBCR complexes directed away from the cell membrane and redistributed among intracellular, post-ER compartments were as or nearly as active as wild-type preBCRs. This was accomplished by appending endomembrane localization sequences to the cytoplasmic tail of a preBCR-forming immunoglobulin mu heavy chain (Ig HC), and testing the resulting "redirected" preBCR activity in preBCR-deficient transformed and primary pro-B cells. These data support the model that preBCRs do not require extracellular ligands at the cell surface for signaling, and that the preBCR is active in post-ER compartments like the Golgi. However, several developmentally important processes linked to preBCR signaling, such as allelic exclusion, could not be evaluated, and it remains possible that these require activities dependent on preBCR surface expression. Therefore, the purpose of the experiments in this small R03 proposal is to build on these findings by evaluating the biological activity of these redirected preBCR complexes in vivo. This will be accomplished in one Aim by establishing transgenic mice that express the redirected Ig HCs and evaluating their ability to support B cell development and effect allelic exclusion in genetically preBCR-deficient and preBCR-sufficient mice. This information will be important to better understand the molecular mechanisms that determine receptor activity in normal immune development and in B cell pathologies. The experiments in this proposal aim to understand how a critical molecule known as the preBCR controls the development of B cells, the cells that make antibodies essential for an immune response. People that are genetically unable to make a preBCR are severely immune deficient. The information obtained is needed to better understand the origin of some B cell leukemias that can be traced to mutations that affect the functioning of the preBCR. [unreadable] [unreadable] [unreadable]